Process of pretreating ferrous metal surfaces before phosphatizing



United States Patent 3,510,365 PROCESS OF PRETREATIN G FERROUS METAL SURFACES BEFORE PHOSPHOTIZING Werner Rausch, Stierstadt, Taunus, Wolfgang Ernst,

Frankfurt am Main, and Gerhard Muller, Hanan am Main, Germany, assignors to Hooker Chemical Corporation, a corporation of New York No Drawing. Filed Nov. 6, 1967, Ser. No. 681,022

Claims priority, application Gtarmany, Nov. 30, 1966,

Int. Cl. cisr 7/10 US. Cl. 148-615 14 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a process for treating metal surfaces and more particularly, it relates to a process for forming improved, finely crystalline, adherent manganese phosphate coatings on ferrous metal surfaces.

In the treatment of metal surfaces, and particularly ferrous metal surfaces, with an aqueous acidic manganese phosphate coating solution, the type of manganese phosphate coating layer formed on the surface, depends to a great extent upon the type of pretreatment used prior to the application of the coating solution. Where the ferrous metal surfaces have been pretreated with a strongly alkaline aqueous cleaning solution, very coarse, crystalline layers are produced. .Even after long periods of phosphatizing, the coatings produced frequently exhibit large areas in which there are no connecting crystalline layers. Similarly, where the ferrous surfaces are first pickled in acid solutions, such as aqueous solutions of hydrochloric or sulfuric acids, very coarse, crystalline manganese phosphate layers are also produced. Where, however, the metal surface is degreased with an organic solvent, such as petroleum solvent, rather than being pretreated with an alkaline cleaner or acid pickling solution, the resulting coating layers have a fine, even, crystalline structure. In many instances, however, it is desirable to have the more thorough cleaning of the ferrous metal surfaces than is obtained with an organic solvent treatment, such as that which is obtained with a strong acid or alkaline solution.

It has further been found that after treatment with a strong acid or alkaline solution, a finely crystalline manganese phosphate coating may be produced where the metal surface is rinsed with organic solvents or wiped with a cloth, prior to the application of the manganese phosphate phosphatizing solution. The former, however, necessitates the use of both an aqueous and organic solvent pretreatment, and hence is undesirable, while the latter method is not readily adaptable to full scale com- 3,510,365 Patented May 5, 1970 mercial processing. Additionally, the pretreatment of the ferrous surfaces by mechanical means, such as sand blasting or blasting with granular wire, will also produce a dense, finely crystalline coating. This method also, is more time consuming, and hence less desirable in a commercial operation, than is a process which uses only chemical treating steps.

In an effort to obtain both the thorough degreasing and cleaning effects of an alkaline cleaning pretreatment and/ or the oxide dissolving properties of an acidic pickle, with the formation of finely crystalline manganese phosphate coating layers, as produced after mechanical pretreatment or organic solvent pretreatment, numerous pretreating steps have been proposed. For example, the alkaline cleaning of the ferrous surfaces has been followed by an intermediate rinse with an aqueous solution containing condensed phosphates, oxolic acid, titanium phosphate, or an alkaline permanganate, or the like. With none of these, however, was it possible to obtain the desired dense, finely crystalline manganese phosphate coating.

It is, therefore, an object of the present invention to provide an improved process for forming a dense, finely crystalline manganese phosphate coating on ferrous surfaces.

A further object of the present invention is to provide an improved pretreatment for ferrous surfaces, prior to the application of a manganese phosphate coating solution, which pretreatment promotes the formation of a dense, finely crystalline manganese phosphate coating on the ferrous surface.

Another object of the present invention is to provide an improved method for treating ferrous metal surfaces whereby a dense, finely crystalline manganese phosphate coating is produced on the surface even after cleaning or pickling of the surface with a strong alkaline or acidic solution.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes a metal treating process wherein a ferrous metal surface is contacted with an aqueous pretreating composition which comprises an admixture of water and manganese-II-orthophosphate, the contact with the pretreating solution being sufficient to effect a conditioning of the ferrous metal surface such that the subsequent application of a manganese phosphatizing solution produces a finely crystalline manganese phosphate coating. Following the conditioning with the aqueous manganese-II-phosphate mixture, the treated surface is then phosphatized with a conventional manganese phosphatizing solution. This process results in the formation of a finely crystalline manganese phosphate coating on the treated metal surfaces, even where the treatment with the aqueous manganese-II-phosphate composition is preceded by an acid or alkaline treatment.

More specifically, in the practice of the present invention, the ferrous metal surfaces to be coated are contacted with an aqueous pretreating composition containing a manganese-II-phosphate. This pretreating composition is an aqueous mixture in which the diflicultly soluble manganese-II-orthophosphate is finely dispersed. The manganese-II-orthophosphates which are used may be prepared in various suitable ways, such as by the alkaline neutralization of phosphoric acid solutions of manganese phosphate, the neutralization being carried out by either the addition of the acid to the base or vice versa, to a pH 3 of about 7.5. Other means of preparing the manganese- II-orthophosphate include the precipitation of this material by the addition of dior tri-sodium phosphate to a phosphate free solution of a manganese salt, such as manganese chloride, manganese carbonate or the like. Preferably, the mol ratio of the manganese to P is within the range of about 120.8 to 0.33, although mol ratios of P 0 which are outside of this range may also be used in many instances. In many instances, it has been found that particularly advantageous results are obtained where at least part of the manganese-II-orthophosphate in the prerinse composition is present as hureaulite. Hurealulite is a manganous phosphate mineral having the general formula H Mn (PO .4H O. Aqueous prerinse mixtures containing hureaulite are found to produce extremely fine crystalline manganese phosphate layers on the subsequent application of the manganese phosphate phosphatizing solution, the crystals of which are generally being sufiiciently fine that they cannot be observed by the naked eye.

It has further been found that the particle size of the manganese-II-orthophosphate in the pretreating solution has an effect on the activation or crystal refining effectiveness which is obtained. Thus, it has been found, that as the particle size of the manganese-II-orthophosphate in the pretreating composition decreases, the activating or crystal refining effect of the pretreatment is increased. Accordingly, in a preferred embodiment of the present invention, the manganese-II-orthophosphate used desirably contains about 50% of material having a particle size below about 3.5 microns and about 90% of a particle size which is less than about 30 microns. In many instances, however, satisfactory results may also be achieved when using manganese-II-orthophosphate of a larger particle size. This has been found to be particularly true where the pretreating mixture is not to be used for an extended period of time, as the particle size of the manganese-II-orthophosphate appears to have more effect on the useful life of the pretreating composition, rather than on the actual activation obtained.

It has also been found that the manganese-II-orthophosphate should be as highly dispersed as possible in the prerinse bath. Thus, it is desirable if the manganese-II-orthophosphate is initially dispersed in the bath by intensive stirring and that this dispersion is, thereafter, maintained by keping the bath in vigorous movement, for example by stirring, pumping, introducing compressed air into the bath, or the like.

The amount of the manganese-II-orthophosphate in the pretreating bath will vary, depending upon the degree of activation or crystal refinement which is desired, greater crystal refinement being obtained as the amount of the manganese-II-orthophosphate in the bath is increased. Typically, amounts of manganese-II-orthophosphate in the bath within the range of about 0.002 to about 5 grams per liter have been found to be preferred. Although amounts of the manganous phosphate in excess of about 5 grams per liter may be used, such larger amounts have not generally been found to result in further improvement of the crystal refinement and, in some instances, may produce sludge deposits upon the metal surfaces treated. Accordingly, it is generally preferred that the pretreating bath contain the manganese-II-orthophosphate in an amount within the range which has been specified.

In addition to the manganese-II-orthophosphate, the pretreating bath desirably may also contain an alkali metal condensed phosphate. By the term alkali metal condensed phosphate it is intended to include the condensed phosphates of sodium, potassium, lithium, cesium, and rubidium. The addition of such condensed phosphate to the pretreating bath are found to aid in obtaining a high degree of dispersion of the manganous phosphate and also to help in preventing settling of the manganous phosphate in the pretreating bath. Of the various alkali metal condensed phosphates which may be used, the tetraalkali metal pyrophosphates are preferred, such as tetrasodium pyrophosphate. Additionally, the tetrasodium pyrophosphate may also be mixed with dialkali metal pyrophosphates, such as disodium pyrophosphate. Desirably, however, the dialkali metal pyrophosphates are not used alone as it can cause phosphate layer formation in the prerinse bath, because of its low pH value. The formation of such a phosphate layer in the prerinse bath may then interfere with the formation of the desired protective manganese phosphate coating in the subsequently applied manganese phosphate coating bath. Higher condensed phosphates, such as the alkali metal tripolyphosphates may also be used in the pretreating bath although such higher phosphates have, generally, been found to be less effective than the pyrophosphates. Where these condensed phosphates are, included in the prerinse bath, they are desirably present in amounts within the range of about 0.5 to 5 grams perliter of the pretreating bath. Additionally, in some instances, the stability of the pretreating bath may be further enhanced by including therein a surface active agent, particularly a non-ionic surface active agent.

The crystal refining effectiveness of the pretreating baths of the present invention may be further improved by also incorporating in the bath a finely distributed, difficultly insoluble iron orthophosphate and/or calcium pyrophosphate. Where such materials are included in the pretreating bath, they are desirably present in amounts comparable to the amounts of the manganese-II-orthophosphates, i.e., amounts within the range of about 0.002 to 5.0 grams per liter. Additionally, like the manganese-II- orthophosphate, it is found that the crystal refining effectiveness of these additives is increased as their particle size is decreased. Accordingly, in their most preferred form, the iron orthophosphate or calcium pyrophosphate preferably has a particle size which is similar to that of the manganese-II-orthophosphate, i.e., 50% of the material has a particle size less than about 3.5 microns and about of the material has a particle size below about 30 microns. As with the manganese-II-orthophosphate, however, satisfactory crystal refining or surface activation may also be obtained in many instances using materials which have a larger particle size than this preferred range.

The prerinse bath of the present invention may be applied to the metal surface in any convenient manner. Thus, for example, the ferrous metal surface may be immersed in the pretreating bath or the bath may be flooded or sprayed onto the metal surface. The pretreating bath, when applied to the metal surface, may be at any suitable temperature, although temperatures between about room temperature (20 C.) and C. are preferred. Generally, rinsing of the metal surface after the application of the pretreating bath is not necessary, although the use of such a rinse has not been found to have any appreciable adverse affect on the activation or crystal refining efiiciency of the pretreatment bath.

Following the application'of the pretreating bath, as has, been described hereinabove, the ferrous metal surfaces are. subjected to the coating action of a manganese phosphate coating solution. Various manganese phosphate phosphatizing solutions, as are known to those in the art, may be utilized and will produce the desired fine crystalline manganese phosphate coating on the metal surface. Typically, these phosphatizing baths are aqueous acidic solutions of manganese phosphate which may contain one or more additives, including oxidizing agents, such as nitrates, nitrites, chlorates, bromates, sulfites, perchlorates, peroxides, permanganates, organic nitro compounds, and the like. Additionally, metal ions, such as nickel, copper, and the like may also be included in the phosphatizing composition. As with the pretreating bath, the manganese phosphate coating composition may be applied in any suitable manner, such as by immersion of the metal surface in the phosphatizing solution. Excellent coatings are also produced, however, by flooding or spraying the phosphatizing solution onto the metal surface.

In the overall process of the method of the present invention, the ferrous metal surfaces to be treated are first subjected to a suitable cleaning operation. Although this cleaning treatment may be of the organic solvent type, the improvement obtained in producing a finely crystalline manganese phosphate coating on the metal surface is most pronounced when the cleaning step includes the use of a strong alkaline cleaner and/or an acid pickel. Thus, in the preferred embodiment of the present method, the ferrous metal surface is first subjected to the action of an alkaline cleaning solution and/or an acid pickling solution, suitable solutions of these types being known to those in the art. Thereafter, the cleaned metal surface is subjected to the action of the pretreating bath of the present invention, for a period sufficient to produce the desired activation of the metal surface. Typically, such treating times will range from a few seconds, e. g., seconds, up to several minutes, e.g. 10 minutes. Other treating times, which will produce the desired activation of the metal surface may, of course, also be used. Following the treatment less than 30 microns, the aqueous dispersion being at a temperature of 90 centigrade and being stirred.

(G) Prerinsing as in (F) but with 2 grams per liter Na P O added to the prerinse bath.

(H) Prerinsing as in (F) but with the prerinse bath containing 1.5 grams per liter of the manganese-II-orthophosphate and 0.5 gram per liter of iron-II-phosphate and 2 grams per liter Na P O (J Prerinsing as in (F) but with the prerinse bath containing 1.5 grams per liter of the manganese-II-orthophosphate, 0.5 gram per liter of dicalcium pyrophosphate and 2 grams per liter Na P O (K) Phosphatizing by immersion for 10 minutes in an aqueous manganese phosphate solution containing 8.85 grams per liter of manganese, 26.8 grams per liter P 0 2.88 grams per liter N0 and 0.25 gram per liter nickel, the phosphatizing solution being at a temperature of 95 degrees centigrade.

Using the above procedures, the treating steps used of the metal surface with the pretreating bath, the surface and the results obtained were as follows:

TABLE Pretreatment steps Cold Sulfuric Cold water-D acidC waterD waterE is then contacted with an aqueous manganese phosphate coating solution for a period sufficient to effect the formation of the desired manganese phosphate coating on the metal surface. As a result of this process, there is consistently produced a ferrous metal surface having an adherent, finely crystalline manganese phosphate coating.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, temperatures are in degrees centigrade and parts and percents are by weight. It is to be appreciated, however, that these examples are merely exemplary of the present invention and are not to be taken as a limitation thereof.

In the examples, steel panels and tempered steel cog wheels were subjected to one or more various pretreating steps and were then phosphatized in an aqueous manganese phosphate coating solution. The treating steps used were as follows:

(A) Degreasing by immersion for 5 minutes in petroleum solvent at 20 centigrade.

(B) Cleaning by immersion for 10 minutes in an alkaline cleaner containing grams per liter NaOH; 25 grams per liter Na Si O and 2.5 grams per liter dodecyl benzene sulfonate, which cleaner was at a temperature of 95 centigrade.

(C) Pickling by immersion for 10 minutes in an aqueous sulfuric acid pickel containing 150 grams per liter H 50 and 3 grams per liter of a pickling inhibitor, the pickel being at a temperature of 60 centigrade.

(D) Rinsing with cold water.

(E) Rinsing with hot water.

(F) Prerinsing by immersion for one minute in an aqueous dispersion containing 2 grams per liter of manganese- II-orthophosphate, as hureaulite, the manganous orthophate containing 36% manganese and 39% P 0 50% of the manganese-II-phosphate having a particle size of less than 3.8 microns and 90% having a particle size of Hot Prerinse Prerinse Prerinse Prerinse F G H J Particle size of manganous phosphate layer From the above results, it is seen that where the manganese-II-orthophosphate containing prerinse baths are utilized, a fine, crystalline manganese phosphate coating is obtained on the surface treated even where the surface is first cleaned with an alkaline cleaner and/ or an acid pickling solution. Moreover, it is seen that these results are comparable to the coatings obtained when organic solvent cleaning is used in place of the alkaline cleaner or acid pickel and that where the pretreating bath of the present invention is not utilized, with the alkaline cleaner or the acid pickel, very coarsely crystalline layers are formed, which layers do not cover the entire metal surface.

While there have been described various embodiments of the invention, the compositions and methods described are not intended to be understood as limiting the scope of the invention as it is realized that changes therein are possible and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same result in substantially the same or equivalent manner, it being intended to cover the invention broadly in Whatever form its principle may be utilized.

What is claimed is:

1. -A method for the treatment of ferrous metal surfaces before phosphatizing with a manganese phosphate coating solution which comprises contacting the ferrous metal surfaces prior to the application of the manganese phosphate coating solution, with an aqueous pretreating mixture, which mixture contains manganese-II-orthophosphate, and maintaining the aqueous pretreating mixture in contact with the surface for a period sufiicient to elfect activation thereof.

2. The method as claimed in claim 1 wherein the manganese-II-orthophosphate is present in the pretreating mixture in an amount within the range of about 0.002 to 5 grams per liter.

3. The method as claimed in claim 2 wherein at least 90% of the manganese-II-orthophosphate is of a particle size of less than about 30 microns.

4. The method as claimed in claim 3 wherein the pretreating mixture also contains an alkali metal condensed phosphate.

5. The method as claimed in claim 4. wherein the alkali metal condensed phosphate is an alkali metal tetrapyrophosphate.

6. The method as claimed in claim 4 wherein the aqueous pretreating mixture also contains iron-II-phosphate.

7. The method as claimed in claim 4 wherein the aqueous pretreating mixture also contains dicalcium pyrophosphate.

8. The method as claimed in claim 4 wherein at least a portion of the manganese-II-orthophosphate in the pretreating mixture is hureaulite.

9. The method as claimed in claim 4 wherein following the treatment with the aqueous pretreating mixture, the treated surface is contacted with an aqueous acidic manganese phosphate coating solution for a period sufficient to effect the formation of a manganese phosphate coating on the surface.

10. The method as claimed in claim 9 wherein prior to the treatment with the aqueous pretreating mixture, the ferrous metal surface is cleaned with an alkaline cleaner.

11. The method as claimed in claim 9 'Wherein prior to the treatment with the aqueous pretreating mixture, the ferrous metal surface is cleaned with an acid pickling solution.

12. A ferrous metal surface having thereon a finely crystalline phosphate coating, formed in accordance with the method of claim 9;

13. A ferrous metal surface having thereon a finely crystalline manganese phosphate coating, formed in accordance with the method of claim 10.

14. A ferrous metal surface having thereon a finely crystalline manganese phosphate coating, formed in accordance with the method of claim 11.

References Cited UNITED STATES PATENTS 2,874,081 2/1959 Cavanagh 1486.l5

RALPH S. KENDALL, Primary Examiner 

